Space

Scientists Find the Hottest Planet in the Universe (So Far)

While astronomers are not going to bother including it in our continuing search for alien life, the planet itself will not be able to look forward to a charmed existence.

An artist's illustration of KELT-9b (right) and its star. Credit: NASA/JPL-Caltech/R. Hurt (IPAC)

An artist’s illustration of KELT-9b (right) and its star. Credit: NASA/JPL-Caltech/R. Hurt (IPAC)

Researchers have discovered the hottest planet ever known, with a surface temperature of 4,327º C – almost as hot as our Sun’s surface. While astronomers are not going to bother including it in our continuing search for alien life, the planet itself will not be able to look forward to a charmed existence. But for as long as it lives, it can teach us a lot. The planet fits into a family of six others known to orbit certain extremely-hot stars, so observing it could tell us how such freakish systems evolve.

Stars in the known universe are broadly classified into seven categories based on their temperature: O, B, A, F, G, K and M. This system was developed by astronomer Cecilia Payne in the mid-20th century and is still in use today, with a few more types added. In this sequence, the stars of O-type are the hottest, with temperatures of nearly 30,000º C. M-type stars are the coolest, with surface temperatures of about 3,500º C, and are also the most common. Our Sun is a G-type star. The hotter stars are relatively harder to find despite being extremely massive (where ‘massive’ is a descriptor of the star’s mass and not size).

We have been hunting for exoplanets for half a century now, and in our database of over 3,600 confirmed candidates, we do not know of a single planet that orbits an O-type or a B-type star. But we have discovered six exoplanets that orbit A-type stars. One explanation for the lack of exoplanet observations surrounding these stars is that the hotter and more massive they are, the faster they spin. This makes them hard to study using the methods and tools at our disposal

Therefore, the only observations we have of them is when they’ve slowed down and cooled down a little. This is the next stage in each star’s life, where it exits the “main sequence”, the first stage of the star. They expand as they cool and eventually swallow planets. This could lead to a lack of observable planets around observable hot stars.

KELT-9 is an A-type star named for the survey that discovered it, the Kilodegree Extremely Little Telescope (KELT). Still in its main sequence, its surface temperature is a mind-boggling 10,000º C (our Sun, for context, is also in its main sequence and is a modest 5,000º C on its surface). In fact, KELT-9 is so hot that astronomers think it fits snugly in the transitional zone between A-type and the even-hotter B-type stars.

The hot star has been found to have an orbiting planet, named KELT-9b, with a surface temperature of of stunning 4,327º C. The planet itself is hotter than an M-type star – making it hotter than most stars we know. It is estimated to be twice the size, and approximately equal to the mass, of Jupiter. The reason this discovery is even more remarkable is that the newly discovered gas giant orbits extremely quick around its host star, completing its journey around the orb in 36 hours.

The hotter a star is, the more rapidly it rotates. And as it whizzes around, it also throws out incredible amounts of ultraviolet radiation. Because KELT-9b is so close, the planet ought to be tidally locked to the star such that the same side faces it at all times. This results in the planet’s dayside being bathed with a glow that can split molecules (if hopes of alien life persisted this far, it is finally time to leave). In fact, astronomers believe that elements like gases and metals found on and around KELT-9b can only exist as atoms, not as molecules. The stellar radiation is also quite likely assiduously stripping away the planet’s atmosphere. We have never directly observed a planet this large and this hot, and so its atmosphere makes for very compelling studies.

Astronomers don’t expect it to be too difficult to obtain detailed observations of the atmosphere of KELT-9b. Currently, ground-based telescopes and the Spitzer and the Hubble space telescopes are all capable of studying the planet’s atmosphere. While Spitzer observes in the infrared wavelength, primarily detecting heat, Hubble peers through the near-infrared to the near-ultraviolet (including the visible) parts of the spectrum. When the James Webb Space Telescope launches next year, it will be able to study the planet’s atmosphere in even better detail in the mid infrared to visible spectrum.

KELT-9 is barely 300 million years old, relatively very young. Our Sun and Solar System are 4.5 billion years old. Once it exits its 500-million-year main sequence, KELT-9 will become a red giant (the hotter the star is, the faster it will burn up its fuel and cools down). And as a red giant, the star will balloon up to twice its current size as well as cool down to 8,000º C. At this point, scientists estimate that all of the planet KELT-9b’s atmosphere will be stripped off completely. Then, the star will move on to its sub-giant phase, becoming four-times its current size and cooling down further to 5,000º C. It will swallow up KELT-9b and finally be at the stage where other A-type stars we observe are today.

Sandhya Ramesh is a science writer focusing on astronomy and earth science.